Mechanism of the Alcohol-Soluble Ionic Organic Interlayer in Organic Solar Cells

Double-Dipole Induced by Incorporating Nitrogen-Bromine Hybrid Cathode Interlayers Leads to Suppressed Current Leakage and Enhanced Charge Extraction in Non-Fullerene Organic Solar Cells

The cathode interlayer plays a vital role in organic solar cells, which can modify the work function of electrodes, lower the electron extraction barriers, smooth the surface of the active layer, and remove solvent residuals. However, the development of organic cathode interlayer lags behind the rapidly improved organic solar cells because their intrinsic high surface tension can lead to poor contact with the active layers. Herein, a double-dipole strategy is proposed to enhance the properties of organic cathode interlayers, which is induced by incorporating nitrogen- and bromine-containing interlayer materials. To verify this approach, the state-of-the-art active layer composed of PM6:Y6 and two prototypical cathode interlayer materials, PDIN and PFN-Br is selected. Using the cathode interlayer PDIN: PFN-Br (0.9:0.1, in wt.%) in the devices can reduce the electrode work function, suppress the dark current leakage, and improve charge extractions, leading to enhanced short circuit current density and fill factor. The bromine ions tend to break from PFN-Br and form a new chemical bond with the silver electrode, which can adsorb extra dipoles directed from the interlayer to silver. These findings on the double-dipole strategy provide insights into the hybrid cathode interlayers for efficient non-fullerene organic solar cells.

Optimized Morphology Enables High-Efficiency Nonfullerene Ternary Organic Solar Cells

Tuning the three-dimensional morphology in the active layer is an effective method to improve the performance of bulk heterojunction organic solar cells (OSCs). In this work, an acceptor-donor-acceptor structured small molecule ST10-CN-1 was synthesized and employed as the guest donor to fabricate ternary OSCs based on a PBDB-T:IT-M host binary system. The incorporation of ST10-CN-1 could broaden the active layer's absorption range of solar light thereby leading to a promotional short-circuit current. Moreover, adding an appropriate amount of ST10-CN-1 could effectively regulate the morphology of the active layer in both the lateral direction and vertical stratification. All of these morphological alterations helped to speed up the exciton dissociation, charge transit, and charge collecting processes, which in turn increased the power conversion efficiency. As a result, an excellent PCE of 11.5% for the ternary device based on PBDB-T:IT-M:ST10-CN-1 was obtained. The enhanced PCE was also linked to the formation of an alloylike state between PBDB-T and ST10-CN-1, as evidenced by the fact that the open circuit voltage of ternary OSCs lay between those for PBDB-T:IT-M (0.925 V) and ST10-CN-1:IT-M (1.064 V). This work illustrates that refining the morphology of the active layer by incorporating an appropriate third component is an effective way to further enhance the device's performance.

Glucose and Its Derivatives as Interfacial Materials for Inverted Organic Solar Cells

Glucose, a widely distributed biomaterial in nature, is presented as a new cathode interfacial material for highly efficient inverted organic solar cells. The interactions between glucose and the indium tin oxide (ITO) substrate as well as the formation mechanisms of the glucose interlayer were investigated by molecular dynamics simulation and relevant experimental tests. The results revealed that the In-OH coordination between the oxygen atom of glucose and the indium of ITO is the key factor for the formation of interfacial dipoles, thereby reducing the work function of the ITO cathode for efficient charge transfer. With PM6:Y6 as the active layer, the power conversion efficiency (PCE) of the organic solar cells was significantly increased from 1.99 to 15.42% after ITO was modified by a glucose interlayer through the traditional spin-coating method. More importantly, glucose can be adsorbed on the ITO surface by a simple immersion process, and the devices based on the modified ITO by immersed glucose achieved a PCE of 14.48%, which is comparable to that of the traditional spin-coating method. Furthermore, we found that the OSCs with the ITO cathodes modified with glucose derivatives including sorbitol and sodium gluconate by different preparation methods also exhibited high performance. The overall performance of the devices with ITO modified by a simple and low-cost immersion method can be maintained at more than 93% of that prepared with the traditional spin-coating method. The results demonstrated that low-price glucose and its derivatives are good candidates as ITO interlayer materials for OSCs, and the effectiveness of the immersion process paves a way for simplifying the manufacture of low-cost and large-area organic solar cells.

High-quality WS2 film as a hole transport layer in high-efficiency non-fullerene organic solar cells

Liquid-exfoliated 2D transition metal disulfides (TMDs) are potential substitutes for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole transport layers (HTLs) in Organic Solar Cells (OSCs). Herein, high-yield and high-quality WS₂ flake layers are prepared by comprehensively controlling the initial concentration, sonication processing time and centrifugal speed. The WS₂ layers deposited on in situ transparent indium tin oxide (ITO) without plasma treatment show higher uniformity and conductivity than that formed on ITO after plasma treatment. With a significant increase in the short-circuit current density (J_SC), the power conversion efficiency (PCE) of PM6:Y6-based non-fullerene OSCs using optimized WS₂ as the HTL is higher than that using PEDOT:PSS as the HTL(15.75% vs. 15.31%). Combining the morphology characteristics with carrier recombination characteristics, the higher quality of the ITO/WS₂ composite substrate leads to better charge transport and a lower bimolecular recombination rate in OSCs, thereby improving the device performance.